Definition
Perihelion is the point of an orbit around the Sun where the orbiting body is closest to the Sun. The word comes from the Greek peri (near) and helios (Sun). The opposite point, where the body is farthest away, is the aphelion. The line joining perihelion and aphelion passes through the Sun and forms the major axis of the ellipse; the Sun sits at one focus of the ellipse, not at its centre.
The general term for the closest point of any orbit is periapsis. It takes a specific name depending on the central body: perigee for orbits around Earth — the Moon and artificial satellites — periastron around another star, and perijove around Jupiter.
Perihelion distance
For an elliptical orbit with semi-major axis a and eccentricity e, the distances from the Sun at perihelion (q) and at aphelion (Q) are:
q = a × (1 − e)Q = a × (1 + e)A perfectly circular orbit (e = 0) has no distinct perihelion because the distance never changes. The more eccentric the orbit, the larger the contrast between the two ends. Comets on elongated orbits show it best: Halley's Comet dives to 0.59 AU from the Sun at perihelion, inside the orbit of Venus, then retreats to about 35 AU, beyond Neptune.
Perihelion and aphelion of the planets
The table below gives the perihelion and aphelion distances of the eight planets in astronomical units (1 AU ≈ 149.6 million km), with the ratio of sunlight received at perihelion compared with aphelion, ((1 + e) / (1 − e))².
| Planet | Eccentricity | Perihelion (AU) | Aphelion (AU) | Sunlight ratio |
|---|---|---|---|---|
| Mercury | 0.2056 | 0.307 | 0.467 | × 2.30 |
| Venus | 0.0068 | 0.718 | 0.728 | × 1.03 |
| Earth | 0.0167 | 0.983 | 1.017 | × 1.07 |
| Mars | 0.0934 | 1.381 | 1.666 | × 1.45 |
| Jupiter | 0.0485 | 4.950 | 5.455 | × 1.21 |
| Saturn | 0.0555 | 9.02 | 10.09 | × 1.25 |
| Uranus | 0.0464 | 18.33 | 20.11 | × 1.20 |
| Neptune | 0.0095 | 29.83 | 30.39 | × 1.04 |
Mercury stands out: at perihelion it receives more than twice as much sunlight as at aphelion. Mars is the other strongly eccentric planet, which is why Mars oppositions that happen near its perihelion — the so-called perihelic oppositions, such as those of 2003 and 2018 — bring the planet much closer to Earth and make it far brighter than average.
Earth's perihelion and the seasons
Earth reaches perihelion every year in early January, usually between 2 and 5 January, at about 147.1 million km (0.983 AU) from the Sun. Aphelion follows six months later, in early July, at about 152.1 million km (1.017 AU).
Many people are surprised that Earth is closest to the Sun during the northern-hemisphere winter. The 3.3% change in distance produces only about a 7% change in the sunlight Earth receives, while the 23.4° tilt of Earth's axis changes the height of the midday Sun and the length of the day far more strongly. The seasons are caused by the tilt of the axis, not by the distance to the Sun.
The distance effect is not zero, though. Because Earth moves faster near perihelion, the northern winter (from the December solstice to the March equinox) is the shortest season, lasting about 89 days, while the northern summer lasts almost 94 days.
Faster at perihelion
Kepler's second law states that the line joining a planet to the Sun sweeps out equal areas in equal times. Near perihelion that line is short, so the planet must move faster to sweep the same area. The vis-viva equation gives the orbital speed v at any distance r, where GM☉ is the gravitational parameter of the Sun:
v² = GM☉ × (2 / r − 1 / a)For Earth this gives about 30.3 km/s at perihelion and 29.3 km/s at aphelion. For Mercury the contrast is much larger: about 59 km/s at perihelion against 39 km/s at aphelion.
A perihelion that moves
The gravitational pull of the other planets slowly turns each orbit within its own plane, so the direction of perihelion drifts over time. This is called apsidal precession. For Earth, combined with the precession of the equinoxes, it makes the date of perihelion move later in the calendar by about one day every 58 years. Around the year 1250, perihelion coincided with the December solstice.
Mercury's perihelion advances by about 574 arcseconds per century relative to the stars. Newtonian perturbations by the other planets explain about 531″ of it. The remaining 43″ per century puzzled astronomers for decades until Einstein explained it in 1915 with general relativity — one of the first successful tests of the theory.
Computing the time of perihelion
Astronomical Algorithms devotes a chapter to the instants when the planets pass perihelion and aphelion. As a first approximation, a planet's mean anomaly M, which increases uniformly with time, is zero at perihelion, so passages repeat every anomalistic period — 365.2596 days for Earth. Real passages differ from this mean schedule because of perturbations. For Earth the actual date can shift by a day or more from one year to the next, mainly because of the Moon: it is the Earth–Moon barycentre, not Earth itself, that follows the smooth ellipse.
The 3D illustration on this page marks the perihelion of each planet on its orbit. Turn on Aphelion in the menu to show both ends of the major axis, and press Play to watch the inner planets speed up as they approach perihelion.
Explore it on AstroAlgos
Frequently asked questions
When does Earth pass perihelion?
Every year in early January, usually between 2 and 5 January, at about 147.1 million km from the Sun. Aphelion follows in early July at about 152.1 million km.
Is Earth warmer at perihelion?
Earth as a whole receives about 7% more sunlight at perihelion than at aphelion, yet the global average temperature is slightly higher in July, near aphelion. The northern hemisphere contains more land, which warms much more in summer than the oceans do.
What is the difference between perihelion and perigee?
Perihelion is the closest point of an orbit around the Sun. Perigee is the closest point of an orbit around Earth, used for the Moon and for artificial satellites.
